1. Field of the Invention
The present invention relates to a digital-to-analog converting circuit, and more specifically to a digital-to-analog converting circuit for use in a display driving circuit of an active matrix display panel.
2. Description of Related Art
Referring to FIG. 1, one typical conventional digital input/analog output type display drive circuit includes a digital-to-analog converter 10 receiving for example a digital signal of seven bits "A" to "G" for generating an analog signal, a selection shift register 12 receiving a sample pulse, and a sample hold circuit array 14 having a plurality of timing inputs connected to respective stages of the selection shift register 12 and a sampling input connected to receive the analog signal, so that a number of sample hold circuits included in the sample hold circuit array 14 sequentially sample and hold the analog signal at the timing of the sample pulse shifted in the selection shift register 12. As a result, the sample hold circuit array 14 supplies a corresponding number of analog signal outputs OUT.sub.1 to OUT.sub.64 which are sampled at respective sequential timings.
Referring to FIG. 2, another typical conventional digital input/analog output type display drive circuit includes a 7-bit latch array 16 including 64 7-bit latches, each of which latches a 7-bit digital input signal at a sequential timing determined by the selection shift register 12. The 7-bit digital input signal latched in each of the latches is compared with a 7-bit digital signal supplied from a 7-bit incrementer 18, by a corresponding magnitude comparator of a magnitude comparator array 20. The magnitude comparator outputs the result of comparison in the form of a pulse width modulation signal to a corresponding sample hold circuit of the sample hold circuit array 14. Each sample hold circuit of the sample hold circuit array 14 samples and holds a ramp voltage signal generated by a ramp wave generator 22, by the pulse width modulation signal supplied from the corresponding magnitude comparator of the magnitude comparator array 20. Thus, each sample hold circuit of the sample hold circuit array 14 generates an analog signal output OUT.sub.1 to OUT.sub.64 corresponding to the digital input signal.
Referring to FIG. 3, there is shown still another typical conventional digital input/analog output type display drive circuit. In this circuit, the 7-bit digital input signal latched in each of the latches included in the 7-bit latch array 16 are converted into an analog signal by a corresponding digital-to-analog converter of a digital-to-analog converter array 24.
The above mentioned display drive circuits have the sample hold circuit in an output ocction, and therefore, can have only a limited precision in the output analog signal, because the output analog signal is influenced by an offset voltage of the sample hold circuit and noise occurring in a switching operation of the sample hold circuit. In particular, in an active matrix liquid crystal display panel very sensitive to precision in the output voltage, if the output analog voltage has a variation of 50 mV to 100 mV, a striped pattern appears in the display screen.
In addition, in the case that a high precision 7-bit digital analog converter is provided for each output as in the example shown in FIG. 3, a required area is large, and it is difficult to obtain a satisfactory precision.
Furthermore, a voltage of 10 V to 20 V is required for driving the liquid crystal display panel. Therefore, a consumed power in the digital-to-analog converting circuit becomes large.
As seen from the above, the conventional digital-to-analog converting circuit could not sufficiently satisfy the demands for the display driving.